CLASS X | SCIENCE | LIGHT
Notes prepared by Subhankar Karmakar
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SIGN CONVENTION FOR SPHERICAL MIRRORS:
According to the new cartesian sign convention:
1. All the distances are measured from pole of the mirror as origin.
2. Distances measured in the same direction as that of incident light are taken as positive.
3. Distances measured against the direction of incident light are taken as negative.
4. Distances measured upward and perpendicular to the principal axis are taken as positive.
5. Distances measured downward and perpendicular to the principal axis are taken as negative.
KEY POINTS TO REMEMBER
• The object is always placed on the left side of the mirror.
• All the distances measured from the pole (P) of mirror to the right side will be considered positive and to the left side will be negative.
• The object distance (u) is always negative.
• If an image is formed behind a concave mirror or to the right side of the mirror, the image distance (v) is positive, if the images formed in front of the mirror or to the left side of the mirror, then the image distance will be negative.
• The image distance (v) for a convex mirror will be always positive.
• The focal length of a concave mirror is always negative
• The focal length of a convex mirror is always positive
• The height of an object is always positive
• If an image is formed above the principal axis its height is positive
• If an image is formed below the principal axis its height is negative
• The height of all the virtual and erect images is positive
• The height of all the real and inverted images is negative.
MIRROR FORMULA:
A formula which gives the relationship between image distance (v), object distance (u) and focal length (f) of a spherical mirror is known as the mirror formula. It is given as
1/v + 1/u = 1/f
Linear magnification produced by mirrors:
The ratio of the height of image to the height of object is known as linear magnification. It is also equal to the ratio of the image distance to the object distance, with a minus sign.
∴ magnification = height of image/height of object
⟹ m = h₂ / h₁ = - v/u
h₁ = height of object
h₂ = height of image
• if the magnification has a plus sign, then the image is virtual and erect.
• if the magnification has a negative sign, then the image is real and inverted.
Position of the image means image distance.
Nature of image means whether the image is "real and inverted" or "virtual and erect".
Size of image means value of magnification.
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